The present invention relates to fluid flow control valves.
In one particular aspect the invention is concerned to provide a valve construction which is amenable to service as a pilot valve controlling the operation of a main water valve in a deluge fire-fighting system for an offshore platform, or in some other marine environment, where the working fluid is seawater. The salt deposits associated with this medium cause problems with valve elements exposed to the water which depend for their operation on sliding contact; such valves can be caused to stick or to seize altogether--with potentially disastrous results in the context of a fire-fighting system. Similar problems can arise in water-irrigation systems. Valves in accordance with the invention avoid such contact and are therefore particularly useful for the control of fluids which leave solid deposits. For the same reason they may also be found particularly useful in the control of corrosive fluids which would otherwise tend to destroy the sealing integrity of sliding valve members. The invention is not, however, restricted to use for the control of these "difficult" fluids and may find general application in the control of liquid and gas flows.
From United Kingdom patent specification no. 988509 there is known a fluid flow control valve comprising a cylindrical housing having axially spaced inlet and outlet ports opening through its wall, an axial slide member provided within the housing to close and open passages between the ports, and a rubber sleeve interposed between the housing and slide member. This sleeve is dimensioned such that a passage connecting the ports is normally free along the inside of the cylindrical housing wall but the slide member is provided with one or more "pistons" (or lands) which expand the surrounding sleeve portion into sealing contact with the facing portion of the housing wall, whereby the passage for fluid flow between the ports can be closed selectively by the movement of the slide member. In a valve of this kind the slide member can therefore be isolated from contact with the fluid by the surrounding rubber sleeve.
Problems can arise in the operation of such a valve, however, owing to the high level of friction between the slide and the rubber sleeve. The movement of the slide will tend to be jerky and require excessive force, and the sleeve may exhibit substantial hysteresis. These problems will be exacerbated by operation at high fluid pressures which will tend to compress the sleeve even more tightly around the slide. In the aforesaid patent specification an embodiment is proposed in which a further sleeve, of a substantially inelastic but flexible material having a lower coefficient of friction than the rubber sleeve, is interposed between the slide and the rubber. This inner sleeve is dimensioned such that its introduction causes a dilation of the outer rubber sleeve and at selected axial positions it has weakened sections provided by a peripheral row of spaced axial slits which will cause circumferential indentations when they are axially remote from supporting lands of the slide. When the slide is moved the lands will straighten said indented sections to assume cylindrical shape and the corresponding sections on the outer sleeve, provided with circumferential beads, will be forced out against the wall of the surrounding housing. The intricate form of this inner sleeve would be expensive to produce, however, and in any event it could only provide a partial solution to the problem of friction. As another solution to this problem, the aforesaid patent specification alternatively proposes to perforate the rubber sleeve to provide an equal fluid pressure on the interior and exterior of the sleeve and thereby reduce the contact pressure with the slide. This expedient, however, destroys the function of the sleeve in isolating the slide from contact with the fluid.